The Nintendo Direct Thread (10th January currently rumoured)

[Flilix]

Jumpin said:

StarDoor said:

Pokemon is already confirmed to release in the second half of 2019. Also, it wouldn't make sense for the most ambitious Pokemon games yet to release sooner after the previous generation than any other Pokemon generation. Three years is the absolute shortest gap that there's been, and Sun/Moon released in November 2016.

Summer is in the second half.

Sun/Moon releasing in November was an abnormality for the franchise.

Let's Go: November 2018
Ultra S/M: November 2017
Sun & Moon: November 2016
ORAS: November 2014
X & Y: October 2013

The last games to be released in summer in Japan were B&W2, but they only got released in October in the west.